Thin contact membrane keyswitches do not offer the extreme high reliability of other types of keyswitches, for example, such as the magnetic core keyswitches of the type shown in U.S. Pat. No. 4,227,163, issued Oct. 7, 1980 to Raymond Barnoski and assigned to Illinois Tool Works Inc. Contact membrane keyswitches, however, do offer sufficient reliability for many applications at an appreciably lower cost. One type of contact membrane keyboard is described in the article entitled "Touch Panel Switches Are Based On Membrane Switch Concept," published in Computer Design, December 1979. In this type of keyswitch, two spaced-apart membrane layers coated with aligned conductive contact lands are separated in an aperture formed in a spacer. When the upper flexible membrane is depressed, contact is made between the upper and lower conductive lands to close the switch. Being a contact switch, however, it does have the inherent deficiencies of all contact switches which include wear, the possibility of oxidation and corrosion of the conductive areas, etc.
While capacitive keyswitches are desirable because of their potential greater reliability and longer life, such keyswitches as these have been designed in the past and have been appreciably more expensive than that of a simple contact membrane switch. Another membrane contact switch is disclosed in U.S. Pat. No. 3,676,607, issued July 11, 1972 to Donald H. Nash et al and assigned on its face to Bell Telephone Laboratories, Incorporated. This keyboard employed field effect transistors, and it was recognized that actual contact between the conductive lands did not have to occur because the field effect transistors could be controlled by capacitance change if contaminants intervened between conductive upper and lower lands. However, the keyswitch of this patent was still designed to be basically a mechanical switch; and moreover, the design led to a split lower land area configuration of two lower lands which resulted in a capacitive keyswitch in which the available capacitive plate area is divided in half and relatively complicated detection circuitry was required.
Another capacitive membrane keyboard is described in an article entitled "Capacitive Membrane Keyboard Bars Contamination," which appeared in Electronic Products Magazine, June 15, 1981 issue. This capacitive keyswitch is designed by Microswitch/Honeywell, and it employs an upper flexible membrane that carries a conductive land above a second membrane which has a second conductive land that is aligned with the upper conductive land. The two conductive lands are positioned in an aperture in a spacer which allows the upper member and land to be deflected towards the lower land until contact is made, thus the actual switching action is of the contact type. On the bottom of the lower membrane, however, there is another conductive area. The conductive coatings on both sides of the lower membrane, therefore, provide a fixed capacitance, while the moveable upper membrane provides a variable capacitance that is in series with the fixed capacitance. Although theoretically the two facing lands at a keyswitch do not have to be brought into contact with each other, in practice, they must be in such a close proximity that the keyswitch will rely on actual contact to operate.
In the present invention, the simple technology of the membrane contact keyswitch may be utilized with the only change required being the provision of a dielectric layer between the upper and lower conductive lands which thereby provides a keyswitch with a high capacitance that has the simple construction of a contact membrane keyswitch.